A classical method of extracting one or more species from a solution has been the use of a solid membrane which is selectively permeable with respect to such species. A solution containing the species is on one side of the membrane and a solution lean with respect to such species on the other. The species diffuse through the membrane by osmosis and the like until the species present in the solutions on the two sides of the membrane reach equilibrium.
Liquid-liquid extraction works on substantially the same principle, but is more flexible. A liquid phase, which is immiscible in another liquid phase from which extraction is to occur (bulk phase), and which contains a material selective towards the species to be extracted is agitated in the bulk phase as a dispersed phase. After a period of contact, the two phases are allowed to separate and the liquid phase containing the extracted species is removed and stripped of the extracted species for recycle.
In the usual operation, a plurality of extraction cells are employed in series with the extractant solution flowing countercurrent to the solution to be operated on, the bulk phase.
There has emerged a new field of technology involving liquid membrane extraction. Broadly, the principle involves surrounding either a liquid phase from which a species is to be extracted or a liquid phase in which species is to be collected by a liquid membrane which is mutually immiscible in the two. The surrounded liquid phase is called the internal phase, the liquid membrane is called the external phase and the internal phase surrounded by the external phase is contacted with a fluid, liquid or gas, from which extraction is to occur. If liquid, the external phase is immiscible in the liquid.
The liquid phase containing the species to be removed, whether the internal or bulk phase, gives up the species to be extracted to the external phase from which it diffuses to the other liquid phase (which is the stripping phase). In the more sophisticated system, the species enters a lean phase and is converted such that the stripping liquid phase remains lean with respect to the extracted species to promote diffusion or species transport and enable higher levels of extraction to occur.
Thus, the liquid membrane (or the organic continuous portion of the emulsion) serves as an intermediary acting in the same way as an immiscible liquid in liquid-liquid extraction, except that it is constantly stripped of the species which it extracts by the liquid stripping phase.
Illustrative of the patents which have issued in the field of development and application of liquid membrane technology include U.S. Pat. Nos. 3,389,078; 3,410,794, 3,454,389; 3,617,546; 3,637,488; 3,650,091; 3,696,028; 3,719,590; 3,733,776; 3,740,315; 3,740,329; 3,779,907, 3,897,308; 3,942,527 and 3,969,265 and reissues thereof, each incorporated herein by reference. In each, Norman Li is named inventor or coinventor.
Of such patents, a patent of particular interest is U.S. Pat. No. 3,779,907. The invention related to a process for removal of the dissolved species from aqueous solutions (bulk phase) which comprises contacting the bulk phase with an emulsion, the emulsion being characterized by having an external phase which is immiscible in the bulk phase and the internal phase, yet permeable to certain dissolved species. The internal phase contains a reactant capable of converting the extracted dissolved species to a non-permeable form. The liquid membrane (external phase) in addition to including an extractant for the species contained in the bulk liquid phase is stabilized by a liquid or liquefied anionic, cationic, or nonionic surfactant.
Systems in which the liquid membrane contains a surfactant in liquid form have, on investigation, exhibited a low rate of extraction and the surfactant is subject to attack by reagents contained in the liquid membrane, the internal stripping phase and/or the bulk phase. This results in leakage of fluid through the liquid membrane with the consequence of partial or total collapse of the emulsion.